This invention relates to an apparatus and method for induction heat sealing lap-seamed tubular paperboard containers to non-metallic closures. The containers are fashioned from paperboard sheet coated at least on its interior forming surface with a layer of a thermoplastic material, such as polyethylene, and also with a metallic foil layer, such as a layer of aluminum foil. The aluminum or other metal foil layer is sandwiched between the polyethylene and the paperboard substrate. Such containers are typically formed by bending a rectangular sheet of the thus laminated material into a tube, the ends of the rectangular sheet being overlapped and sealed together. This construction defines two exterior corners and two interior corners of the tube.
Typically, a non-metallic closure to close one or both ends of the container is fashioned in the form of a plug having a central disc with an upstanding, peripheral skirt. The skirt portion is in surface contact with an interior surface portion of that end of the container which is covered by the closure. In affixing such an end closure to a tubular container, electrical induction heating has sometimes been employed. An induction coil is placed in the proximity of that end of that container which is to be closed, with the end closure positioned within the container end. The induction coil is now energized with the result that eddy currents are induced in the metallic foil coating or layer of the paperboard laminate, the eddy currents causing the usual heating in the metal foil thereof. This in turn causes the polyethylene layer on the interior surface of the tubular paperboard container to partially melt and thereby fuse with the adjacent plastic surface of the end closure which is in surface contact with a portion of the container interior.
Such a method of induction heat sealing yields, however, an incomplete seal between the skirt of the disc closure and the adjacent interior container wall. Experience has shown that eddy currents tend to avoid the corners of the rectangular laminate which forms the tubular container. Accordingly, workers in this art have never been able to obtain an induction heat seal for such a closure to the inside surface at the corner portions of the lap-seamed area of such a laminated paperboard container. At the lap-seam area, two thin aluminum foil surfaces (being the ends of the metal foil coating) are separated by both paperboard and the polyethylene coating. The metal foil coating of a lapped seam tubular container thus defines an extremely thin-walled split sleeve. In theory, this sleeve cannot be completely induction-heated by eddy currents because a complete circuit for eddy current flow is not provided at all regions of the sleeve. The theoretical background for this eddy current behavior is set forth in a text entitled Basics of Induction Heating by Chester A. Tudbury, at page 118. Other portions of this text which are relevant to this invention appear at pages 12 and 13, also incorporated herein by reference.